This invention relates generally to paper handling equipment, and more particularly to equipment which feeds cut paper sheets from stacks of such sheets.
In equipment that feeds cut paper sheets from stacks of sheets, it is not unusual for the cut sheets to be misfed or for more than one sheet to be fed at one time (“multifeed”). In either case, such an occurrence may result in jamming or other faulty operation of the equipment.
The purpose for a paper feeder is to feed a single sheet of paper from a stack and to leave all other sheets remaining in the stack. To insure that at least one sheet is fed, the drive force on top of the sheet must exceed the retarding force elsewhere on the sheet. To prevent more than one sheet from feeding, the retarding forces on all sheets below the top sheet must be higher than the drag force exerted on the second sheet by the first sheet as it is being fed. For these reasons, the forces between sheets in a stack affect the performance of a feeder.
A number of factors may increase the likelihood of a misfeed or multifeed. One factor is the presence of so-called “edge welds” in the stack of cut paper sheets from which feeding is to be performed. Edge welds may occur during the paper manufacturing process during the step of cutting reams of paper prior to wrapping the reams. Large sheets of paper may be assembled into relatively large stacks of typically 500 sheets and then cut to a standard size such as letter size using a guillotine cutter. If the blade of the cutter is less than optimally sharp, individual sheets in the stack may be slightly bent downward at the edges as cutting takes place. The bending of the sheets at the edges may occur under very high local pressure, so that the edges of two or more sheets may be temporarily affixed to one another.
Another factor that may cause misfeeds or multifeeds is bonding between sheets other than at the edges. Such bonding may result during storage and transportation of wrapped reams of paper. For example, ten reams are commonly packed in a carton, and a considerable number of cartons may be stacked up during shipment and/or storage. The resulting load on the reams of the lower cartons in the stack of cartons may amount to hundreds of pounds. This loading may tend to press air out from between adjacent sheets in the reams and may lead individual paper fibers in neighboring sheets to interlock with each other. Both of these effects tend to form a bond between the surface of one sheet and the surface of a neighboring sheet. These bonds may cause the forces between adjacent sheets in a stack to be substantially higher than ordinary sheet-to-sheet friction and thereby cause misfeeds and/or multifeeds.
A third factor which may contribute to multifeeds arises from interfaces between reams in a stack of cut paper sheets. When a stack of paper in a paper tray is partially depleted, it is not unusual for an operator of the paper handling equipment to replenish the paper supply by adding additional paper to the top of the stack. This may occur when the stack is nearly completely depleted. The sheets within the original stack of sheets, as well as the sheets within the added stack, may have relatively high sheet-to-sheet coefficients of friction due to the edge welding and sheet-to-sheet bonding factors described in the previous two paragraphs. However, the coefficient of friction between the top sheet of the original stack and the bottom sheet of the added stack may be much lower than for the sheets above and below this interface. This is because the two sheets at the interface were not cut together or packed together so that there is little intermeshing of fibers. Also, when the operator adds new paper to the stack, a partial layer of air may be trapped between the new and old stacks at the interface.
Given such an interface, a problem may arise when the sheet feeder has fed most of the newly added sheets and then attempts to feed one of the few remaining sheets above the ream interface. In particular, the combination of high sheet-to-sheet friction for the few sheets above the interface and the relatively low sheet-to-sheet friction at the interface creates a relatively high probability that several sheets will be fed simultaneously. In other words, the singulation function of the feeder is more likely to fail at the ream interface by feeding more than one sheet just above the ream interface.
Still another factor that may lead to misfeeds and/or multifeeds may be present in stacks of pre-printed sheets, in which the ink printed on the sheets may tend to produce a mild adhesive effect between adjacent sheets.
It is a common practice for skilled operators of paper handling equipment to bend or fan a ream of paper before loading in order to break edge welds, break the bonds between interlocked fibers, and add a layer of air between adjacent sheets of a ream. Such conditioning of the ream may reduce sheet-to-sheet friction between the sheets in the ream, thereby reducing the chance of misfeeds and/or multifeeds. However, many operators are not aware of the desirability of bending or fanning a ream of paper before loading, and fail to do so. Also, some operators may have relatively small hands, and so may not be able to easily handle a ream in order to condition the ream before loading in a paper tray.